Sedimentary Rocks

Sedimentary rocks form when mud and sand are deposited in layers on the Earth’s surface. The layers are deposited in many environments including oceans, rivers and deserts. These layers of sand and mud are later buried. The weight of overlying layers compresses the mud and sand to form solid rock. Because sedimentary rocks begin to form at the Earth’s surface, they tell us about ancient landscapes.


Basic Information About Sedimentary Rocks

Sedimentary rocks are deposited in layers, usually underwater, with the oldest layers on the bottom and the youngest layers on the top.

One important feature of sedimentary rocks is the size of the grains that form them. The names of these grains depend upon their size.

The grain size tells us about the energy of the currents in the environment of deposition. Large grains of gravel and sand are deposited by faster currents that are strong enough to move the larger particles. These currents may be found in stream channels, on ocean beaches or in windy deserts. So, sandstones indicate a high energy environment.

The finer, silt and clay size grains that make up mudstones were deposited in quieter water such as in the deep ocean or a lake or swamp.

Sandstones tend to be harder and more resistant than mudstones. So sandstones usually form cliffs and ledges. Mudstones form soft, crumbly, covered slopes.

Sedimentary rocks are the only type of rocks that contain fossils. Fossils give us a great deal of information. They tell us about the environment when the sediments were deposited. For example, if we find coral fossils in the rock, it was most likely deposited in sea water because corals live in marine environments.

If we find dinosaur fossils we look at the type of dinosaur fossils as well as plant pollens and other paleontological clues. This helps us to determine that the rock was deposited on or near land because most dinosaurs were terrestrial animals that walked on land.


Sedimentary Rocks of Devils Tower National Monument

Three sedimentary rock layers are exposed at Devils Tower National Monument—the Spearfish Formation, the Gypsum Springs Formation and the Sundance Formation. When you visit Devils Tower you can see these layers by hiking along the Red Beds trail in the monument.

The Spearfish Formation

The oldest rocks in the monument are the sedimentary rocks of the Spearfish Formation. This formation is named after the town of Spearfish, South Dakota. These are the bright, red-colored layers exposed at lower elevations near the Belle Fourche River valley.

The Spearfish Formation was deposited during Triassic time—approximately 240 million years ago. The oldest dinosaur fossils are found in Triassic age rocks!

During Triassic time, North America, South America and Africa were joined together as a single large continent near the equator. The Appalachian Mountains of the eastern US were very young. The Atlantic Ocean and Gulf of Mexico had not yet formed.

The Spearfish Formation is made of mudstones and very fine grained sandstones. It was deposited in a fairly low energy environment on a flat, hot desert plain near the ocean similar to some of the environments near the Persian Gulf in the Middle East today. The red color comes from iron oxides or “rust” present in the sediments. The iron was oxidized in a hot, dry desert climate as the sediments would be moistened, then dried over and over again causing them to “rust”. The frequent white layers are the mineral gypsum, the chalky stuff that many walls are made of. It is a type of evaporite mineral similar to salt. It forms when salty water evaporates in the heat, leaving a white crust behind.

Not exactly a vacation paradise!!!!!

Gypsum Springs Formation

The Gypsum Springs Formation appears as a thin white layer above the Spearfish Formation. It is not very well exposed. It is formed almost entirely of the mineral gypsum which is very soft and easy to erode.

The Gypsum Springs Formation is Jurassic age—a time period in which dinosaurs ruled the earth about 150 million years ago.

During Jurassic time, North America, South America and Africa were beginning to drift apart forming the young Atlantic Ocean and Gulf of Mexico.

The mineral gypsum forms when saline waters, such as those in a shallow ocean or desert lake, evaporate in hot, dry climates. The Gypsum Spring Formation is thought to have formed on an ancient tidal flat in an arid climate.

Numerous dinosaur tracks have been found in the Gypsum Springs Formation west of the monument. To learn more, go to:

Sundance Formation

The Sundance Formation contains the youngest sedimentary rocks in the monument. It is composed of both mudstones and sandstones. In the picture below, the yellow colored Hulett Sandstone layer of the Sundance Formation forms a cliff above the gray, covered slope formed in the underlying mudstones of the Stockade Beaver layer.

The Sundance Formation is also Jurassic age and is about 150 million years old. The continents of Africa and South America were pulling away from North America to form the young Atlantic ocean and Gulf of Mexico.

The Sundance Formation was deposited in a shallow, ancient sea called the Sundance Sea. It covered much of the Rocky Mountain area during this time.

The Hulett Sandstone layer was deposited in shallow water near the shoreline where you can see the sandy beach. The underlying mudstones of the Stockade Beaver layer were deposited in the quieter water farther offshore.

The Sundance Formation contains more fossils than any other layer in the monument. These fossils consist of oyster shells and, occasionally, belemnites—the remains of fossil squid.

To the west, the Sundance Formation contains numerous dinosaur tracks which can be visited at the Red Gulch Dinosaur Tracksite area administered by the Bureau of Land Management--

The tracks formed when dinosaurs walked across a shallow, tidal flat covered with algal mats and ripple marks. The Sundance Formation at the monument also contains ripple marks however, no dinosaur tracks have yet been found.


The Geology of the Monument from 150 to 55 million years ago

Over the next 100 million years, over 7000 feet of younger sedimentary rocks were deposited above the Sundance Formation.

These sedimentary rock layers can still be found to the west in the Powder River Basin. At Devils Tower they have all been removed by later erosion so they are not discussed here.

Finally, the stage is set for the Tower itself to appear….



Devils Tower Main Index

Geologic Overview

Sedimentary Rocks (current section)

Igneous Rocks




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